Electronic systems may be made up of different circuits which must communicate with each other. The different circuits may be implemented on a single chip or may be implemented in different chips or packages each mounted on a common substrate. Whether the different circuits which must communicate with each other are implemented on a single chip or separate chips on a common substrate such as a printed circuit board, the physical connection which allows signals to be communicated between the circuits is commonly referred to as a communications bus or simply a “bus.” Such a bus may include a single transmission path made of an electrically conductive material, or may include multiple electrically conductive transmission paths which allow multiple signals to be transmitted simultaneously between the circuits. The electrical signals may be transmitted in a single direction on a given transmission path or in both directions on a transmission path.
FIG. 1 shows a diagrammatic representation of two circuits 100 and 101 connected by three separate buses for communicating electrical signals between circuits. The bus shown in dashed box 102 provides unidirectional communications from circuit 100 to circuit 101. The bus shown in dashed box 103 provides unidirectional communications from circuit 101 to circuit 100. Dashed box 104 shows a bidirectional bus that allows electrical signals to be communicated from circuit 100 to circuit 101 and from circuit 101 to circuit 100. For purposes of this example, each bus includes four transmission paths, each transmission path indicated by reference numeral 105. It will be appreciated that the four transmission paths 105 in each bus are shown only for purposes of example and that a given communications bus may include fewer or many more transmission paths. It will also be appreciated that more than two circuits may be connected for communication across a common bus. For example, a microprocessor may have internal buses shared by several functional units.
As with all circuit elements, the transmission paths which are included in a communications bus may be subject to manufacturing defects or errors. For example, the electrically conductive material making up a transmission path may not be formed or deposited properly on a semiconductor substrate or printed circuit board leaving a gap or opening at some point along the conductive material. In such a case, the transmission path cannot carry the desired signals because it does not provide the required electrical continuity. Another type of error arises due to the common requirement that numerous transmission paths be located very close together, either on a semiconductor substrate or on a printed circuit board. Occasionally, the electrically conductive material making up two different transmission paths in a bus may touch, causing a short circuit between the two transmission paths. In this case, the shorted transmission paths cannot provide the intended transmission function in which both paths carry independent electrical signals. Also, a transmission path in a bus may inadvertently be formed so that it makes contact with a conductor carrying the system supply voltage or the system ground. In this type of bus error the affected transmission path is said to be “stuck” since the transmission path is held continuously at either the supply voltage level or ground.